1. Field of the Invention
The present invention relates to a developing device for the electrophotographic copying machine by use of one-component developer.
2. Description of the Prior Art
Generally, there exists a one-component developer type not employing a mixture of carrier, particles and toner particles and instead employing only toner without carrier in addition to a two-components developer type employing both of them, as to the developing system.
In a developing device employing one-component toner of high inherent resistance value per volume, as it is called, where the resistance value is higher than the medium, there is an improved developer carrying member (developing roller) which performs the actual development in opposition to the photosensitive body. This is to improve the image quality such as halftone, sharpness, etc..
For instance, according to the Laid-open Japanese Patent Publication No. 55-46768, a resistor layer is formed on the surface of the electrically conductive supporter layer of the developing roller. The above-mentioned laid-open patent provides a developing device having a resistance ratio value of the resistor layer in the range of 10.sup.8 to 10.sup.15 .OMEGA./cm, in which the intensity of electric field is controlled in the developing area. Namely, it is desirable to use a conductive developing roller to perform bias developing by use of the developing roller. However, in such construction, an edge effect cannot be exhibited. On the contrary, in the case of using a completely insulated developing roller, a bias effect cannot be exhibited and thereby background dirt may be present. Therefore, the resistance value of the resistor layer has to be raised within the area that has the developing bias applied thereto on the developing roller.
Further, the Laid-open Japanese Patent Publication No. 57-66456 proposes an electric current supplying device, in which a medium-resistance-value resistor layer is formed on the high-resistance-value resistor layer of the developing roller, and thereby the developing electric current is supplied to the developing roller through the high-resistance-value layer and thereafter through the medium-resistance-value layer. Namely, in the above-mentioned laid-open patent, it is the main (primary) object to optimize the intensity of the developing electric field by suitably setting the resistance ratio and the thickness of the material employed for the resistor layer, and further, the self-bias effect of the medium-resistance-value resistor layer is utilized in the developing area.
In case that, by utilization of the prior-art technology, only high-resistance-value resistor layer is used on the surface layer of the developer carrying member in order to obtain an edge effect at the time of developing, and further, the thickness of the toner layer on the developing roller is regulated to a desired value by use of a regulation member for regulating the thickness of the developer disposed on the developer carrying member before entering the developing area, the following problems may occur.
Namely, there exists an opposing electric charge counterbalanced with the toner layer on the surface of the developing roller. Therefore, in general, the developing portion is in a state of excessive opposing electric charge immediately after the thickness regulation member which regulates the thickness of the thin layer which passes therethrough. On such occasion, since the effective bias to be applied to the developing roller in the developing process varies due to the surface electric field caused by the amount of electric charge on the surface of the developing roller, the amount of the effective bias needs to be balanced with that of the electric charge of the developer layer entering the developing area, in order to perform a proper developing operation in the developing area. As to the conventional device, the above-mentioned matter cannot be guaranteed.
Furthermore, regarding the developing roller having high-resistance-value resistor layer as mentioned above, the opposing members such as the photosensitive body and the toner removing roller perform a toner supplying operation in a state of keeping an electric charge after finishing the developing process. Consequently, the toner supply varies sometimes. For instance, the toner layer on the surface of the developing roller has a portion peeled off in the developing process and other portions remain without playing a role in development. These conditions exist in the developing device, so that it causes an unequal electric potential on the developing device. Consequently, the above matter yields a difference in the amount of toner supplied. This causes unevenness in the toner layer and thereby exerts a bad influence on the performance of development.